Dc-dc converter control circuit and dc-dc converter including same

ABSTRACT

A DC-DC converter control circuit, to control a DC-DC converter having an inductor and two switches, including a first feedback circuit; a second feedback circuit; a synthesis circuit to add a first feedback voltage indicating a DC component of an inductor current based on an output voltage of the DC-DC converter and a second feedback voltage indicating an AC component thereof to generate a third feedback voltage; a comparator to compare the third feedback voltage with a reference voltage to output a comparison result; and an on-time adjusting circuit to adjust on/off time of the switches based on the comparison result for outputting a control signal depending on the adjusting result. The second feedback voltage is generated based on a difference between input and output voltages of the DC-DC converter when the control signal is low and based on the output voltage when the control signal is high.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is based on and claims priority pursuant to 35U.S.C. §119 to Japanese Patent Application No. 2011-199580, filed onSep. 13, 2011 in the Japan Patent Office, the entire disclosure of whichis hereby incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates to a DC-DC converter control circuit tocontrol a DC-DC converter to moderate fluctuation in an output voltageof the DC-DC converter, and a DC-DC converter including the DC-DCconverter control circuit

2. Description of the Related Art

At present, as mobile devices have become more sophisticated and morecompact, switching power supplies using hysteresis control that operatesat high frequency, which is efficient and can be made compact, is widelyused.

For example, a DC-DC converter that operates while generating a constantoutput voltage is known, as illustrated in FIG. 1. FIG. 1 is a blockdiagram illustrating a configuration of a conventional DC-DC converter10X. In FIG. 1, the DC-DC converter includes a switch SW101, a diodeD101, an inductor L101, capacitors C101 and C102, resistors R101 throughR105, a buffer amplifier AMP101, a comparator CMP101, and a referencevoltage source Vref. When an input voltage Vin is applied to an inputterminal (switch SW 101), the DC-DC converter generates an outputvoltage Vout to a load 101.

The DC-DC converter 10X shown in FIG. 1 generates a ramp signal using alow-pass filter 107 connected to an input voltage Vin side of theinductor L101 and adds the ramp signal to a feedback voltage via thebuffer circuit (amplifier) AMP101 to control hysteresis.

In general, in DC-DC converters, an inductor current flowing through theinductor includes a ripple component. It is preferable that the ripplecomponent be within a range of from 10% to 20% of the inductor current(e.g., 100 mA). However, in the DC-DC converter 10X shown in FIG. 1, asthe voltage at the input side of the inductor L101 is proportional tothe input voltage Vin, the operating frequency fluctuates greatly.Therefore, the ripple component of the output voltage Vout alsofluctuates greatly.

In addition, the DC-DC converter shown in FIG I needs the buffer circuitAMP101 to obtain a gain 1. Therefore, the operating frequency of theDC-DC converter 10X depends on characteristics of the buffer circuitAMP101 and the buffer circuit needs to carry out phase compensation.Therefore, essentially the operating frequency cannot be increased alot.

Thus, the output voltage of the DC-DC converter is affected by thefluctuation in the strength of the ripple component of the inductorcurrent. Therefore, in the DC-DC converter, in order to reduce thefluctuation in the output voltage of the DC-DC converter, it isnecessary to keep the strength of the ripple component of the inductorcurrent constant.

In addition, the output voltage of the DC-DC converter is also affectedby the fluctuation in the frequency of the ripple component of theinductor current. Therefore, in the DC-DC converter, in order to reducethe fluctuation in the output voltage of the DC-DC converter, it isnecessary to keep the frequency of the ripple component of the inductorcurrent Constant as well.

BRIEF SUMMARY

In one aspect of this disclosure, there is a provided novel DC-DCconvert control circuit to control a DC-DC converter including a powersupply terminal to which an input voltage is input, a ground terminal,an output terminal to output an output voltage, a first switchingelement and a second switching element connected in series between thepower supply terminal and the ground terminal, an inductor connectedbetween the output terminal and a junction node between the firstswitching element and the second switching element, a first capacitorconnected between the output terminal and the ground terminal. The DC-DCconverter control circuit includes a first feedback circuit, a secondfeedback circuit, a synthesis circuit, a reference voltage generatorcircuit, a first comparator, an on-time adjusting circuit, and a drivercircuit. The first feedback circuit detects an output voltage of theDC-DC converter, and generates a first feedback voltage indicating adirect-current component of an inductor current flowing through theinductor of the DC-DC converter based on the output voltage. The secondfeedback circuit generates a second feedback voltage indicating analternating-current component of the inductor current flowing throughthe inductor of the DC-DC converter based on the input voltage and theoutput voltage of the DC-DC converter. The synthesis circuit adds thefirst feedback voltage to the second feedback voltage to generate athird feedback voltage. The reference voltage generator circuitgenerates a predetermined first reference voltage corresponding to adesired output voltage of the DC-DC converter. The first comparatorcompares the third feedback voltage the first reference voltage tooutput a comparison result signal indicating the comparison result. Theon-time adjusting circuit adjusts on-time (length of time on) andoff-time (length of time off) of the switching elements based on thecomparison result signal, and outputs either a high control signal or alow control signal based on the adjusting result to feed back to thesecond feedback circuit. The driver circuit controls the switchingelements so that when the control signal is low, the first switchingelement is switched on and the second switching element is switched off,and when the control signal is high, the first switching element isswitched off and the second switching element is switched on. The secondfeedback circuit operates in accordance with the control signal from thefirst comparator, and generates the second feedback voltage based on adifference between the input voltage and the output voltage of the DC-DCconverter when the control signal is low and generates the secondfeedback voltage based on the output voltage of the DC-DC converter whenthe control signal is high.

In another aspect of this disclosure, there is a provided novel DC-DCconverter including the power-supply terminal to which an input voltageis input, the ground terminal, the output terminal to output an outputvoltage, the first switching element and a second switching elementconnected in series between the power supply terminal and the groundterminal, the inductor connected between the output terminal and ajunction node between the first switching element and the secondswitching element, the first capacitor connected between the outputterminal and the ground terminal, and the above-described DC-DCconverter control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other aspects, features, and advantages arebetter understood by reference to the following detailed descriptionwhen considered in connection with the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating a configuration of a conventionalDC-DC converter;

FIG. 2 is a block diagram illustrating a configuration of a DC-DCconverter according to a first embodiment of this disclosure;

FIG. 3 is a block diagram illustrating a configuration of a referencevoltage generator circuit shown in FIG. 2;

FIG. 4 illustrates operation of a ripple signal generator circuit whenthe control signal is high;

FIG. 5 illustrates the operation of the ripple signal generator circuitshown in FIG. 4 when the control signal is low;

FIG. 6 illustrates the operation of a variation of a ripple signalgenerator circuit according to a variation shown in FIG. 4 when thecontrol signal HYSO is low;

FIG. 7 illustrates the operation of the ripple signal generator circuitshown in FIG. 6 when the control signal HYSO is high;

FIG. 8 is a block diagram illustrating a configuration of an on-timeadjusting circuit shown in FIG. 2;

FIG. 9 is a timing chart illustrating waveforms of signals n the DC-DCconverter shown in FIG. 2;

FIG. 10 is a block diagram illustrating a configuration of a DC-DCconverter according to a variation of the first embodiment;

FIG. 11 is a block diagram illustrating a configuration of a DC-DCconverter according to a second embodiment;

FIG. 12 is a block diagram illustrating a configuration of an on-timeadjusting circuit in the DC-DC converter shown in FIGS. 11; and

FIG. 13 is a timing chart illustrating waveforms in respective signal inthe DC-DC converter shown in FIG. 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In describing preferred embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected, and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner and achieve a similar result. Referring now to thedrawings, wherein like reference numerals designate identical orcorresponding parts throughout the several views, particularly to FIGS.2 through 13, DC-DC converters according to illustrative embodiments isdescribed.

First Embodiment

FIG. 2 is a block diagram illustrating a configuration of a DC-DCconverter 10 according to the present embodiment of this disclosure. TheDC-DC converter 10 according to the first embodiment operates to reducefluctuation in an output voltage; more particularly, it operates to keepthe strength of a ripple component of an inductor current of an inductorconnected to the output terminal constant.

In FIG. 2, the DC-DC converter 10 includes two switching elements M1 andM2 connected in series between a power-supply input terminal(power-supply terminal) 4 connected to a voltage source and a groundterminal 0, a converter control circuit I to control the switchingelements M1 and M2, an output inductor L1 having one end connected to ajunction node LX between the switching elements M1 and M2, and an outputcapacitor C1 connected between a junction node VL of the other end ofthe inductor L1 and the ground terminal 0. The output terminal 2 of theDC-DC converter 10 is connected to a load 3.

Each of the switching elements M1 and M2 is constituted by, for example,power transistors. The switching element M1 functions as a power switch,and the switching element M2 functions as a rectification switch.

The converter control circuit 1 includes a reference voltage generatorcircuit 11, a comparator 12, a driver circuit 13, a feedback voltagegenerator circuit 14, a ripple signal generator circuit (second feedbackcircuit) 15, a synthesis circuit 16, and an on-time adjusting circuit17.

The feedback voltage generator circuit 14 includes resistors R1 and R2connected in series between the output terminal 2 and the groundterminal 0 and a capacitor C2 connected in parallel to the resistor R1.The feedback voltage generator circuit 14 detects an output voltageVOUT, and the resistors R1 and R2 divide the detected output voltageVOUT to the synthesis circuit 16.

The synthesis circuit 16 includes capacitors C11 and C12. The capacitorC11 is electrically grounded, and removes the high-frequency componentfrom the divided voltage to generate a first feedback voltage VFB1 (tobe described later). The feedback voltage generator circuit 14 and thecapacitor C11 operate together as a first feedback circuit to generatethe first feedback voltage VFB1. The first feedback voltage VFB1corresponds to a direct-current (DC) component of an inductor current ILacross the inductor L1.

An input voltage VIN, an output voltage VOUT from the output terminal 2,and a control signal HYPO output from the comparator 12 are input to theripple signal generator circuit 15, and the ripple signal generatorcircuit 15 generates a voltage signal (ripple signal) having a waveformhomothetic to a waveform of the ripple component of the inductor currentIL, as a second feedback voltage VFB2. The second feedback voltage VFB2corresponds to an alternating-current (AC) component of the inductorcurrent IL.

An input voltage VIN, an output voltage VOUT from the output terminal 2,and a control signal HYSO output from the comparator 12 are input to theripple signal generator circuit 15, and the ripple signal generatorcircuit 15 generates a voltage signal (ripple signal) having a waveformhomothetic to a waveform of the ripple component of the inductor currentIL, as a second feedback voltage VFB2. The second feedback voltage VFB2corresponds to an alternating-current (AC) component of the inductorcurrent IL.

By composing the second feedback voltage VFB2 with the first feedbackvoltage VFB1 via the capacitor C12 of the synthesis circuit 16, a thirdfeedback voltage VFB3 is generated. The third feedback voltage VFB3 isinput to a non-inverting input terminal (+) of the comparator 12.

A reference voltage VREF generated in the reference voltage generatorcircuit 11 is input to an inverting input terminal (−) of the comparator12.

The comparator 12 compares the third feedback voltage VFB3 with thereference voltage VREF and generates a comparison result signal CMPOindicating whether or not the third feedback voltage VFB3 is higher thanthe reference voltage VREF, and sends the comparison result signal CMPOto the on-time adjusting circuit 17.

The comparison result signal CMPO, the input voltage VIN, and the outputvoltage VOUT, and a control signal HYSO output from the on-timeadjusting circuit 17 itself are input to the on-time adjusting circuit17. The on-time adjusting circuit 17 adjusts on-time and off-time of theswitching elements M1 and M2 (i.e., the length of time the switchingelements M1 and M2 are on and off, respectively), and outputs either ahigh or a low control signal HYSO depending on the results of theadjustment. The control signal HYSO is used to control the overalloperation of the converter control circuit 1.

The control signal HYSO is input to the ripple signal generator circuit15, the on-time detection circuit 17, and the driver circuit 13. Inaccordance with the control signal HYSO, the driver circuit 13 generatesa first driving signal SET1 to control of the switching element M1 and asecond driving signal SET2 to control of the switching element M2.

When the control signal HYSO is low, the switching element M1 isswitched on, and the switching element M2 is switched off. Conversely,when the control signal HYSO is high, the switching element M1 isswitched off and the switching element M2 is switched on.

FIG. 3 is a block diagram illustrating a configuration of the referencevoltage generator circuit 11 shown in FIG. 2. The reference voltagegenerator circuit 11 includes a voltage control circuit 21 and adigital-analog (D/A) converter (DAC) 22. The voltage control circuit 21controls the D/A converter 22 in accordance with an external setpointsignal to generate a predetermined reference voltage (first referencevoltage) VREF. The setpoint signal indicates a desired current of theinductor current IL corresponding to a desired output voltage of theDC-DC converter 10, and the reference voltage VREF is generated as avoltage corresponding to the desired voltage value.

In accordance with the setpoint signal, when the inductor current ILflowing through the inductor L1 is increased, the reference voltagegenerator circuit 11 increases the reference voltage VREF, and when theinductor current is decreased, the reference voltage generator circuit11 decreases the reference voltage VREF. Accordingly, the inductorcurrent IL is varied in accordance with the desired current value, andthus, the output voltage of the DC-DC converter 10 can be varied toobtain a desired value.

When the switching element M1 is switched on, and the switching elementM2 is switched off, the inductor current IL essentially varies inaccordance with a voltage difference between the input voltage VIN andthe output voltage VOUT according to the following formula:

dIL/dt=(VIN−VOUT)/L1   (1)

On the other hand, when the switching element M1 is switched off and theswitching element M2 is switched on, the inductor current IL essentiallyvaries in accordance with the output voltage VOUT according to thefollowing formula:

dIL/dt=(−VOUT)/L1   (2)

Accordingly, the ripple component of the inductor current IL haswavelengths determined by the formulas 1 and 2, in accordance with thestates of the switching elements M1 and M2. Based on the input voltageVIN and the output voltage VOUT, the ripple signal generator circuit 15generates a current having a waveform homothetic to a waveform of theripple component of the inductor current IL, and then generates thesecond feedback voltage VFB2 having a waveform homothetic to a waveformof the current.

FIGS. 4 and 5 are block diagrams illustrating the configuration of theripple signal generator circuit 15. FIG. 4 illustrates operation of theripple signal generator circuit 15 when the control signal HYSO is low,and FIG. 5 illustrates the operation when the control signal HYSO ishigh.

The ripple signal generator circuit 15 includes switching elements M31and M32, resistors R31 and R32, and an inverter INV31. The switchingelement M31 and M32 are connected in series between the power supplyinput terminal 4 (input voltage VIN) and the ground terminal 0 (groundvoltage VGND). The first resistor R31 and the second resistor R32 areconnected in series between a junction node between the switchingelements M31 and M32 and a terminal through which the output voltageVOUT is input. The second feedback voltage VFB2 is generated at ajunction node N31 between the first resistor R31 and the second resistorThe generated second feedback voltage VFB2 is sent to the synthesiscircuit 16

Switching of the switching element M31 is controlled by the controlsignal HYSO, and switching of the switching element M32 is controlled bythe control signal HYSO inverted by the inverter INV31.

While the control signal HYSO is low (FIG. 4), the switching element M31is on and the switching element M32 is off. By contrast, while thecontrol signal HYSO is high (FIG. 5), the switching element M31 is offand the switching element M32 is on. Namely, the switching element M31operates in conjunction with the switching element M1, and the switchingelement M32 operates in conjunction with the switching element M2.

In a state in which the switching element M31 is on and the switchingelement M32 is off (see FIG. 4), a current I1 proportional to a value[(VIN−VOUT)/(R31+R32)] flows inside the ripple signal generator circuit15, and the second feedback voltage VFB2 proportional to the value ofthe current I1 is generated at the junction node N31. In a state inwhich the switching element M31 is off and the switching element M32 ison (see FIG. 5), a current I2 proportional to a value[(−VOUT)/(R31+R32)] flows inside the ripple signal generator circuit 15,and the second feedback voltage VFB2 proportional to the value of thecurrent I2 is generated at the junction node N31.

Accordingly, the ripple signal generator circuit 15 generates a currenthaving a waveform homothetic to the waveform of the ripple component ofthe inductor current IL and generates the second feedback voltage VFB2having a waveform homothetic to the waveform of the current.

The third feedback voltage VFB3 made synthetically from the firstfeedback voltage VFB1 and the second feedback voltage VFB2 has awaveform homothetic to the waveform of the ripple component of theinductor current IL. The value of the third feedback voltage VFB3 isincreased in proportional to the voltage difference [VIN−VOUT] when thecontrol signal HYSO is low and is decreased in proportional to theoutput voltage VOUT when the control signal HYSO is high.

In addition, capacitances of the capacitors C11 and C12 are selectedbased on the following formulas, corresponding to an average frequency“f” of the second feedback voltage VFB2 and the third feedback voltageVFB3.

f=a/C11+b/C12   (3)

Herein, “a” and “b” are certain coefficients assumed to be approximatelyconstant. When the average frequency “f” is increased, the capacitancesof the capacitors C11 and C12 are decreased so as to satisfy formula 3.When the average frequency is decreased, the capacitances of thecapacitors C11 and C12 are increased so as to satisfy formula 3.

(Variation of Ripple Signal Generator)

FIGS. 6 and 7 are block diagrams illustrating a configuration of aripple signal generator circuit 15A according to a variation of theripple signal generator circuit 15. FIG. 6 illustrates the operation ofthe ripple signal generator circuit 15A when the control signal HYSO islow, and FIG. 7 illustrates the operation when the control signal HYSOis high.

The ripple signal generator circuit 15A includes a capacitor (thirdcapacitor) C31 instead of the resistor (second resistor) R32 in theripple signal generator circuit 15 shown in FIGS. 4 and 5.

In a state in which the switching element M31 is on and the switchingelement M32 is off (see FIG. 6), a current I3 proportional to a value[(VIN−VOUT)/R31] flows inside the ripple signal generator circuit ISA,and a second feedback voltage VFB2-A proportional to the value of thecurrent I3 is generated at a junction node N31 between the resistor R31and the capacitor C31. In a state in which the switching element M31 isoff and the switching element M32 is on (see FIG. 7), a current I4proportional to a value [(−VOUT)/R31] flows inside the ripple signalgenerator circuit ISA, and the second feedback voltage VFB2-Aproportional to the value of the current 14 is generated at the junctionnode N31.

Accordingly, the ripple signal generator circuit 15A generates a currenthaving a waveform homothetic to the waveform of the ripple component ofthe inductor current IL and generates the second feedback voltage VFB2-Ahaving a waveform homothetic to the waveform of the current.

Herein, to make synthetically with the second feedback voltage VFB2-Aand the first feedback voltage VFB1, the capacitances of the capacitorsC12 and C31 satisfies the relation “C31 <<C12.”

FIG. 8 is a block diagram illustrating a configuration of the on-timeadjusting circuit 17. The on-time adjusting circuit 17 includes variablecurrent sources 41 and 42, an inverter INV4, a switching element (thirdswitching element) M41, a comparator (second comparator) 43, a capacitor(second capacitor) C41, and a reference voltage source (second referencevoltage generator) 44 to generate a reference voltage (second referencevoltage) VREF1. The inverter INV41 converts a comparison result signalinto an inverting signal INVO. The switching element M41 operates inaccordance with the inverting signal INVO (and thus with the comparisonresult signal CMPO). The second capacitor C41 is connected between anon-inverting input terminal (+) of the comparator 43 and the groundterminal 0. A voltage across the capacitor C41, that is, a voltage at aterminal of the capacitor C41 connected to the non-inverting inputterminal (+) of the comparator 43, is a capacitor voltage VMON. Thecomparison result signal CMPO, the input voltage VIN, the output voltageVOUT, and the control signal HYSO are input to the on-time adjustingcircuit 17.

The switching element M41 connects the first variable current source 41to the capacitor C41 while the inverting signal INVO is low, that is,for a time period during which the third feedback voltage VFB3 is higherthan the reference voltage VREF. Conversely, the switching element M41connects the second variable current source 42 to the capacitor C41while the inverting signal INVO is high, that is, for a time periodduring which the third feedback voltage VFB3 is lower than the referencevoltage VREF.

The first variable current source 41 obtains the difference [VIN−VOUT]between the input voltage VIN and the output voltage VOUT when thecontrol signal HYSO falls. Then, the first variable current source 41sends a current based on the obtained difference [VIN−VOUT] to thesecond capacitor C41 while the third feedback voltage VFB3, arrivingimmediately after the control signal HYSO falls, equals or is higherthan the first reference voltage VREF (while the inverting signal INVOis low).

The second variable current source 42 obtains the output voltage VOUTwhen the control signal HYSO rises. Then, the second variable currentsource 42 sends a current based on the obtained output voltage VOUT tothe second capacitor C41 while the third feedback voltage VFB3, arrivingimmediately after the control signal HYSO rises, is lower than the firstreference voltage VREF (while the inverting signal INVO is high).

When the inverting signal INVO is low, the current flowing from thefirst variable current source 41 to the capacitor C41 is proportional tothe voltage difference [VIN−VOUT] between the input voltage VIN and theoutput voltage VOUT. When the inverting signal INVO is high, the currentflowing from the first variable current source 41 to the capacitor C41is proportional to the output voltage VOUT.

The capacitor voltage VMON of the capacitor C41 changes depending onwhether the first variable current source 41 or the second variablecurrent source 42 is connected. That is, the change in the capacitorvoltage VMON depends on the current flowing through the variable currentsources 41 and 42 and the capacitance of the capacitor C41.

VMON=VREF1−ΔV   (4)

ΔV=I/C41   (5)

1∝(VIN−VOUT)   (6)

When the current is sent from the first variable current source 41 tothe capacitor C41, the capacitor voltage VMON is proportional to thevoltage difference [VIN−VOUT]. When the current is sent from the secondvariable current source 42 to the capacitor C41, the capacitor voltageVMON is proportional to the output voltage [VOUT].

The comparator 43 compares the capacitor voltage VMON with the secondreference voltage VREF1. When the capacitor voltage VMON becomes higherthan the second reference voltage VREF1, the comparator 43 switches thecontrol signal HYSO from low to high. When the capacitor voltage VMONbecomes lower than the second reference voltage VREF1, the comparator 43switches the control signal HYSO from high to low.

The voltage value of the second reference voltage VREF1 is determined bythe current values of the variable current sources 41 and 42 and thecapacitance of the capacitor C41.

Next, operation of the DC-DC converter 10 shown in FIG. 2 is describedbelow with reference to FIG. 9. FIG. 9 is a timing chart illustratingwaveforms of the signals in the DC-DC converter 10. Herein, thereference voltage VREF is set at a predetermined constant value based onthe setpoint signal, and FIG. 9 illustrates the operation of thecurrent, signals, voltages in the DC-DC converter 10 when the referencevoltage VREF is set at the predetermined constant value.

In FIG. 9, while the inverting signal INVO is low (for example, timeperiod from t1 to t2), the capacitor voltage VMON is increased. Then,when the capacitor voltage VMON becomes higher than the second referencevoltage VREF1, the comparator 43 switches the control signal HYSO fromlow to high. At this time, as described above, the switching element M1is switched off, the switching element M2 is switched on, and therefore,the inductor IL starts flowing from the ground terminal 0 to theinductor L1 via the switching element M2. As the time has elapsed, theinductor current IL is decreased, and the third feedback voltage VFB3 isdecreased.

While the inverting signal INVO is high (for example, time period t2 tot3), the capacitor voltage VMON is decreased. Then, when the capacitorvoltage VMON becomes lower than the second reference voltage VREF1, thecomparator 43 switches the control signal HYSO from high to low. At thistime, the switching element M1 is switched on and the switching elementM2 is switched off, and therefore, the input voltage VIN is applied tothe inductor L1. As a time has elapsed, the inductor current IL isincreased, and the third feedback voltage VFB3 is increased. Then,above-described operation is repeated.

When the control signal HYSO is low, the inductor current IL is changedin accordance with the voltage difference [VIN−VOUT] (see formula I).When the control signal HYSO is high, the inductor current changed inaccordance with the output voltage [−VOUT] (see formula 2). In addition,when the control signal HYSO is low, the third feedback voltage VFB3 ischanged in accordance with the voltage difference [VIN−VOUT]. When thecontrol signal HYSO is high, the third feedback voltage VFB3 is changedin accordance with the output voltage [−VOUT].

Accordingly, in the DC-DC converter 10, the DC-DC converter controlcircuit 1 adjusts the control signals HYSO so that a length of the timeperiod during which the control signal HYSO is low is proportional tothe value of the voltage difference [VIN−VOUT], and a length of the timeperiod during which the control signal HYSO is high is proportional tothe value of the output voltage [VOUT].

In order to make the length of the time period during which the controlsignal HYSO is low proportional to the value of the voltage difference[VIN−VOUT], the capacitor voltage VMON can be set proportional to thevoltage difference [VIN−VOUT]. In order to make the length of the timeperiod during which the control signal HYSO is high proportional to thevalue of the output voltage VOUT, the capacitor voltage VMON can be setproportional to the output voltage VOUT.

The current values generated by the variable current sources 41 and 42are determined by considering the gradient of the third feedback voltageVFB3.

As a result, the lengths of the time periods during which the controlsignal HYSO is low and during which the control signal HYSO is highensure that the fluctuation range of the third feedback voltage VFB3 isrestricted within the voltage range VHYS. Accordingly, the fluctuationrange of the strength of the ripple component of the inductor current ILcan be restricted within a predetermined range (see FIG. 9).

As described above, the DC-DC converter 10 according to the presentembodiment operates to keep the strength of the ripple component of theinductor current IL, which can reduce the fluctuation in the outputvoltage.

In the present embodiment, the DC-DC converter 10 acquires the DCcomponent and the AC component of the inductor current IL separately,and make synthetically the DC component and the AC component to generatethe third feedback voltage VFB3. Thus, the third feedback voltage VFB3can accurately follow the fluctuation in the ripple component of theoriginal inductor current IL.

(Variation of Converter Control Circuit)

FIG. 10 is a block diagram illustrating a configuration of a DC-DCconverter 10-1 according to a variation of the first embodiment.

In a converter control circuit 1A shown in FIG. 10, when the switchingelement M1 is on and the switching element M2 is off, the convertercontrol circuit 1A detects an indication that a reverse current flowsfrom the output terminal 2 to the ground terminal 0 via the inductor L1and the switching element M2 and prevents the reverse current fromflowing. More specifically, in addition to the components of theconverter control circuit 1 shown in FIG. 2, the converter controlcircuit 1A further includes a comparator 51 (third comparator) to detectthe reverse current. An inverting input terminal (−) of the comparator51 is connected to the junction node LX, and a non-inverting inputterminal (+) thereof is connected to the ground terminal 0.

When the switching element M2 is on, the comparator 51 compares avoltage at the junction node LX with the ground voltage VGND, anddetermines whether or not a voltage difference caused by a reversecurrent is generated. When a reverse current flows, the comparator 51sends a reverse-current detection signal BKO to a driver circuit 13A.

In addition to the operation of the driver circuit 13 shown in FIG. 2,the driver circuit 13A shown in FIG. 10 switches both switching elementsM1 and M2 off when the reverse-current detection signal BKO is inputfrom the comparator 51 to the driver circuit 13A. Even when thecomparator 51 detects the reverse current and the switching elements MIand M2 are switched off, the ripple signal generator circuit 15 keepsgenerating the second feedback voltage VFB2 having a waveform homotheticto the waveform of the ripple component of the inductor current IL.

Accordingly, when the reverse current disappears, the driver circuit 13Acan restart controlling the switching elements M1 and M2 in accordancewith the control signal HYSO. In this variation, in addition to theeffect of the DC-DC converter 10 shown in FIG. 2, the DC-DC converter10-1 shown in FIG. 10 can prevent the adverse effects caused by reversecurrents.

Second Embodiment

FIG. 11 is a block diagram illustrating a configuration of a DC-DCconverter 10-2 according to a second embodiment. The DC-DC converter10-2 according to the present embodiment operates to reduce fluctuationin the output voltage, more particularly; it operates to keep thefrequency of the ripple component of the inductor current of theinductor connected to the output terminal constant.

In FIG. 11, the converter control circuit 1B includes an on-timeadjusting circuit 17A including a constant current source 61, instead ofthe on-time adjusting circuit 17 in the converter control circuit shownin FIG. 2. The on-time adjusting circuit 17A does not reference to theinput voltage VIN, the output voltage VOUT, and the control signal HYSO.

FIG. 12 is a block diagram illustrating a configuration of the on-timeadjusting circuit 17A shown in FIG. 1. In FIG. 12, the on-time adjustingcircuit 17A includes a constant current source 61, an inverter INV41, aswitching element (third switching element) M61 to operate in accordancewith the inverting signal INVO (corresponding to the comparison resultsignal CMPO), a comparator (second comparator) 43, a capacitor (secondcapacitor) C41, and a reference voltage source (second reference voltagesource) 44.

The constant current source 61 is connected to the terminal on the sideof the capacitor C41 that is not grounded. The switching element M61 isconnected between the ground terminal 0 and the terminal on the side ofthe capacitor C41 that is not grounded. The switching element M61 isswitched off in a time period during which the inverting signal INVO islow, that is, the third feedback voltage VFB3 is higher than thereference voltage VREF. The switching element M61 is switched on toshort out (connected to the ground terminal) in a time period duringwhich the inverting signal INVO is high, that is, the third feedbackvoltage VFB3 is lower than the reference voltage VREF. Accordingly, thecapacitor C41 is electrically charged while the inverting signal INVO islow, and is discharged while the inverting signal INVO is high.

Next, operation of the DC-DC converter 10-2 is described below withreference to FIG. 13. FIG. 13 is a timing chart illustrating waveformsof signals in the DC-DC converter 10-2.

A constant current is applied from the constant current source 61 to thecapacitor C41. Therefore, the capacitor C41 repeats charging anddischarging, and the capacitor voltage VMON is periodically changed.

While the inverting signal INVO is low (for example, the time periodfrom t1 to t2 shown in FIG. 13), the capacitor voltage VMON isincreased. Then, when the capacitor voltage VMON becomes higher than thesecond reference voltage VREF1, the comparator 43 switches the controlsignal HYSO from low to high.

While the inverting signal INVO is high (for example, the time periodfrom t2 to t3), the capacitor voltage VMON is decreased. Then, when thecapacitor voltage VMON becomes lower than the second reference voltageVREF1, the comparator 43 switches the control signal HYSO from high tolow. Then, above-described operation is repeated.

The capacitances of the capacitors C11 and C12 are selected to satisfyformula 3, assuming the frequency of the ripple component of theinductor current IL is “f”.

As described above, the DC-DC converter 10-2 according to the secondembodiment can operate to keep the frequency of the ripple component ofthe inductor IL constant, and thus, which can reduce the fluctuation inthe output voltage.

The respective converter control circuits 1, 1A, and 1B can beintegrated on a single integrated circuit (IC). Alternatively, theswitching elements M1 and M2 can he formed as a part of the integratedcircuit constituting the converter control circuits 1, 1A, and 1B. Yetalternatively, the variation of the first embodiment with reference toFIGS. 6, 7, and 10 can he combined with the configurations of the secondembodiment shown in FIG. 11.

In the DC-DC converter 10, 10-1 and 10-2 according to the presentdisclosure, the ripple signal generator circuit IS operates depending onthe control signal HYSO. When the control signal HYSO is low, the secondfeedback voltage VFB2 is generated based on the voltage difference[VIN−VOUT], and when the control signal HYSO is high, the secondfeedback voltage VFB2 is generated based on the output voltage [VOUT].

Accordingly, whether the DC-DC converter 10 operates in discontinuousconduction mode (DCM) or continuous conduction mode (CCM), the secondfeedback voltage VFB2 having a waveform homothetic to the waveform ofthe ripple component (PWM signal) of the inductor current IL can bereliably generated.

That is, the third feedback voltage VFB3 is generated by obtaining thefirst feedback voltage VFB1 corresponding to the DC component of theinductor current IL and the second feedback voltage VFB2 correspondingto the AC component of the inductor current IL separately, and thensynthesizing the first feedback voltage VFB1 and the second feedbackvoltage VFB2, which allows the third feedback voltage VFB3 to accuratelyfollow to the fluctuation in the ripple component of the originalinductor current IL.

In above-described embodiments, the ripple signal generator circuit 15operates depending on the voltage difference [VIN−VOUT]. Therefore, evenwhen both input voltage VIN and the output voltage VOUT fluctuate, thestrength of the ripple component of the inductor current IL (firstembodiment) or the frequency of the ripple component of the inductorcurrent IL (second embodiment) can be kept constant. Accordingly, in acase in which an inductor having a different inductance is used, theDC-DC converter of the present disclosure can stably operate.

In addition, the operating frequency of the DC-DC converter of thepresent disclosure can be increased with stable operation.

Moreover, the DC-DC converter control circuit according to the presentdisclosure can carry out peak-current compensation.

As described above, the DC-DC converter control circuit causes the DC-DCconverter to operate so that the strength or the frequency of the ripplecomponent of the inductor current can kept constant, which can reducefluctuation in the output voltage of the DC-DC converter. In the presentdisclosure, the DC-DC converter can be provided having one of theabove-described DC-DC converter control circuits.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that, withinthe scope of the appended claims, the disclosure of this patentspecification may be practiced otherwise than as specifically describedherein.

1. A DC-DC converter control circuit to control a DC-DC converterincluding a power supply terminal to which an input voltage is input, aground terminal, an output terminal to output an output voltage, a firstswitching element and a second switching element connected in seriesbetween the power supply terminal and the ground terminal, an inductorconnected between the output terminal and a junction node between thefirst switching element and the second switching element, and a firstcapacitor connected between the output terminal and the ground terminal,the DC-DC converter control circuit comprising: a first feedback circuitto detect an output voltage of the DC-DC converter; and generate a firstfeedback voltage indicating a direct-current component of an inductorcurrent flowing through the inductor of the DC-DC converter based on theoutput voltage; a second feedback circuit to generate a second feedbackvoltage indicating an alternating-current component of the inductorcurrent flowing through the inductor of the DC-DC converter based on theinput voltage and the output voltage of the DC-DC converter; a synthesiscircuit to add the first feedback voltage to the second feedback voltageto generate a third feedback voltage; a reference voltage generatorcircuit to generate a predetermined first reference voltagecorresponding to a desired output voltage of the DC-DC converter; afirst comparator to compare the third feedback voltage with the firstreference voltage to output a comparison result signal indicating thecomparison result; an on-time adjusting circuit to adjust on-time(length of time on) and off-time (length of time off) of the switchingelements based on the comparison result signal, and output either a highcontrol signal or a low control signal based on the adjusting result tofeed back to the second feedback circuit; and a driver circuit tocontrol the switching elements so that when the control signal is low,the first switching element is switched on and the second switchingelement is switched off, and when the control signal is high, the firstswitching element is switched off and the second switching element isswitched on, wherein the second feedback circuit operates in accordancewith the control signal from the first comparator, and generates thesecond feedback voltage based on a difference between the input voltageand the output voltage of the DC-DC converter when the control signal islow and generates the second feedback voltage based on the outputvoltage of the DC-DC converter when the control signal is high.
 2. TheDC-DC converter control circuit according to claim 1, wherein theon-time adjusting circuit comprises: a first variable current source; asecond variable current source; a second comparator; a second capacitorhaving a first end connected to a non-inverting input terminal of thesecond comparator and a second end connected to a ground terminal; athird switching element to operate in accordance with the comparisonresult signal, connectable between the first end of the second capacitorand the first variable current source or between the first end of thesecond capacitor and the second variable current source; and a secondreference voltage generator circuit, connected between an invertinginput terminal of the second comparator and the ground terminal, togenerate a second reference voltage; wherein the on-time adjustingcircuit operates in accordance with the input voltage, the outputvoltage, and the control signal, the third switching element connectsthe first variable current source to the first end of the secondcapacitor while the third feedback voltage is equal to or higher thanthe second reference voltage, and connects the second variable currentsource to the first end of the second capacitor while the third feedbackvoltage is lower than the second reference voltage, the first currentsource obtains the difference between the input voltage and the outputvoltage when the control signal falls, and sends a current based on theobtained difference to the second capacitor while the third feedbackvoltage, arriving immediately after the control signal falls, is equalto or higher than the first reference voltage, the second current sourceobtains the output voltage when the control signal rises, and sends acurrent based on the obtained output voltage to the second capacitorwhile the third feedback voltage, arriving immediately after the controlsignal rises, is lower than the first reference voltage, and the secondcomparator compares a voltage of the second capacitor with the secondreference voltage, switches the control signal from low to high when thevoltage of the second capacitor becomes higher than the second referencevoltage, and switches the control signal from high to low when thevoltage of the second capacitor becomes lower than the second referencevoltage.
 3. The DC-DC converter control circuit according to claim 1,wherein the on-time adjusting circuit comprises: a constant currentsource; a second comparator; a second capacitor having a first endconnected to the constant current source and a non-inverting inputterminal of the second comparator and a second end connected to a groundterminal; a third switching element, to operate in accordance with thecomparison result signal, connectable between the first end of thesecond capacitor and the ground terminal; and a second reference voltagegenerator circuit, connected between an inverting input terminal of thesecond comparator and the ground terminal, to generate a secondreference voltage; wherein the third switching element is switched offwhile the third feedback voltage is equal to or higher than the secondreference voltage and is switched on to short out while the thirdfeedback voltage is lower than the second reference voltage, and thesecond comparator compares a voltage of the second capacitor with thesecond reference voltage, switches the control signal from low to highwhen the voltage of the second capacitor becomes higher than the secondreference voltage, and switches the control signal from high to low whenthe voltage of the second capacitor becomes lower than the secondreference voltage.
 4. The DC-DC converter control circuit according toclaim 1, wherein the second feedback voltage circuit comprises: a firstresistor and a second resistor connected in series to each other,wherein when the control signal is low, the input voltage of the DC-DCconverter is applied to one end of the connected resistors, and theoutput voltage of the DC-DC converter is applied to the other end of theconnected resistors, when the control signal is high, the output voltageis applied to one end of the connected resistors, and the other end ofthe connected resistors is connected to ground; the second feedbackvoltage circuit generates the second feedback voltage at a junction nodebetween the first resistor and the second resistor.
 5. The DC-DCconverter control circuit according to claim 1, wherein the secondfeedback voltage circuit comprises: a first resistor and a thirdcapacitor connected in series to each other, wherein when the controlsignal is low, the input voltage of the DC-DC converter is applied to aresistor side of the connected resistor and capacitor, and the outputvoltage of the DC-DC converter is applied to a capacitor side of theconnected resistor and capacitor, when the control signal is high, theoutput voltage is applied to the resistor side of the connected resistorand capacitor, and the capacitor side of the connected resistor andcapacitor is connected to ground; the second feedback voltage circuitgenerates the second feedback voltage at a junction node between thefirst resistor and the third capacitor.
 6. The DC-DC converter controlcircuit according to claim 1, wherein the second feedback circuitcomprises a ripple signal generator circuit to detect a ripple signal inaccordance with a ripple current flowing through the inductor of theDC-DC converter for output as the second feedback voltage.
 7. The DC-DCconverter control circuit according to claim 1, further comprising: athird comparator to detect a reverse current flowing from the outputterminal to the ground terminal via the inductor and the secondswitching element when the first switching element is switched on,wherein the driver circuit control the switching elements so that theboth first switching element and the second switching element areswitched off when the reverse current is detected.
 8. The DC-DCconverter control circuit according to claim 1, wherein the firstfeedback circuit detects the output voltage and divides the outputvoltage to generate the first feedback voltage.
 9. ADC-DC convertercomprising: a power supply terminal to which an input voltage is input;a ground terminal; an output terminal to output an output voltage; afirst switching element and a second switching element connected inseries between the power supply terminal and the ground terminal; aninductor connected between the output terminal and a junction nodebetween the first switching element and the second switching element; afirst capacitor connected between the output terminal and the groundterminal; and a DC-DC converter control circuit comprising: a firstfeedback circuit to detect an output voltage of the DC-DC converter, andgenerate a first feedback voltage indicating a direct-current componentof an inductor current flowing through the inductor based on the outputvoltage; a second feedback circuit to generate a second feedback voltageindicating an alternating-current component of the inductor currentflowing through the inductor based on the input voltage and the outputvoltage of the DC-DC converter; a synthesis circuit to makesynthetically the first feedback voltage and the second feedback voltageto generate a third feedback voltage; a reference voltage generatorcircuit to generate a predetermined reference voltage corresponding to adesired output voltage of the DC-DC converter; a first comparator tocompare the third feedback voltage with the first reference voltage tooutput a comparison result signal indicating the comparison result; anon-time adjusting circuit to adjust on-time and off-time of theswitching elements based on the comparison result signal for outputtingeither a high control signal and a low control signal based on theadjusting result to feed back to the second feedback circuit; and adriver circuit to control the switching elements so that when thecontrol signal is low, the first switching element is switched on andthe second switching element is switched off, and when the controlsignal is high, the first switching element is switched off and thesecond switching element is switched on, wherein the second feedbackcircuit operates in accordance with the control signal, and the secondfeedback circuit generates the second feedback voltage based on thedifference between the input voltage and the output voltage when thecontrol signal is low and generates the second feedback voltage based onthe output voltage when the control signal is high.
 10. The DC-DCconverter according to claim 9, wherein the first feedback circuit ofthe DC-DC converter control circuit detects the output voltage anddivides the output voltage to generate the first feedback voltage.